PHYSICAL REVIEW C
VOLUME 53, NUMBER 6
JUNE 1996
Evidence of initial state interactions in multinucleon pion absorption
D. Androić, 9 G. Backenstoss, 1 D. Bosnar, 9 H. Breuer, 4 H. Döbbeling, 8 T. Dooling, 7 M. Furić, 9 P. A. M. Gram, 3
N. K. Gregory, 5 A. Hoffart, 2,8 C. H. Q. Ingram, 8 A. Klein, 7 K. Koch, 8 J. Köhler, 1 B. Kotliński, 8 M. Kroedel, 1 G. Kyle, 6
A. Lehmann, 1 A. O. Mateos, 5 K. Michaelian, 8 T. Petković, 9 R. P. Redwine, 5 D. Rowntree, 5 U. Sennhauser, 8
N. Šimičević, 5 R. Trezeciak, 2 H. Ullrich, 2 M. Wang, 6 M. H. Wang, 6 H. J. Weyer, 1,8 M. Wildi, 1 and K. E. Wilson 5
~LADS Collaboration!
1
University of Basel, CH-4056 Basel, Switzerland
University of Karlsruhe, D-76128 Karlsruhe, Germany
3
LAMPF, Los Alamos, New Mexico 87545
4
University of Maryland, College Park, Maryland 20742
5
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
6
New Mexico State University, Las Cruces, New Mexico 88003
7
Old Dominion University, Norfolk, Virginia 23529
8
Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
9
University of Zagreb, HR-41001 Zagreb, Croatia
~Received 1 February 1996!
2
The absorption of 239 MeV positive pions on 3 He, 4 He, N, and Ar is studied. A strong enhancement is seen
in the differential cross sections in the kinematic region associated with free p -p scattering. This enhancement
is interpreted as a clear signature of initial state interactions in the absorption of pions on nuclei. @S05562813~96!50306-7#
PACS number~s!: 25.80.Ls, 13.75.Gx, 25.10.1s
The study of pion absorption in nuclei remains a subject
of considerable interest in medium energy physics, primarily
because the absorption process frequently appears to involve
more than two nucleons. It had long been expected that absorption on two nucleons would be the dominant process of
sharing the energy and momentum of the pion. Two-nucleon
absorption has indeed been observed to be important, but
absorption on more than two nucleons also appears to play a
significant role @1,2#. The role of nucleon rescattering in raising the final-state particle multiplicity has been largely understood @3,4#, but that of an initial state interaction ~ISI! of
the incident pion, which based on the free p -N and N-N
cross sections should be a significantly larger effect, has not
been established.
In a simple picture of ISI, a pion scatters off a nucleon
before being absorbed on a nucleon pair, and the originallyscattered nucleon has a correlation between its energy and
angle given by two-body scattering kinematics. Compared to
scattering from a free proton, this correlation is smeared out
by the Fermi motion of the nucleon. Early searches for evidence of ISI were not conclusive @5–8#, but such a correlation has recently been observed in absorption on 3 He and
quantitatively identified as an ISI signature @9# by comparison with simulations. In this paper we extend the identification from 3 He to 4 He, N, and Ar for the absorption of positive pions of 239 MeV kinetic energy.
The experiment was performed at the Paul Scherrer Institute in Villigen, Switzerland using the large acceptance detector system ~LADS!. The LADS detector is a cylindrical,
nonmagnetic detector that includes two layers of multiwire
proportional chambers ~MWPCs! followed by three layers of
plastic scintillator. These detector elements provide particle
0556-2813/96/53~6!/2591~3!/$10.00
53
identification, energy resolution, and tracking capability for a
wide dynamic range of medium-energy charged particles.
The inclusion of endcaps allows the detector to cover over
98% of 4p solid angle. The energy threshold of the detector,
determined mainly by energy losses in the carbon fiber walls
of the high-pressure gas target and in the MWPCs, is below
25 MeV for protons. A more complete description of the
detector is available elsewhere @10#.
For this paper we have focused on p p p final states. In
order to enhance the strength of the ISI signal in the data
sample it was required that three protons and no other energetic particles be detected, and that each of the protons have
more than 30 MeV kinetic energy. This largely removed reactions in which one of the protons was a spectator or was
emitted in an evaporation process. In addition, it was required that the unobserved energy be less than 50 MeV in
order to exclude most reactions in which undetected nucleons participated. This last restriction was especially important to enhance the visibility of the ISI signal from N and Ar.
Once a sample of events was selected, the correlation between energy and angle was plotted for each of the three
detected protons. The results are shown in Fig. 1 for 3 He,
4
He, N, and Ar. The salient feature on the plots is the prominent enhancement in the lower left hand sector, which for
3
He has been identified as being due to the recoil proton
from ISI @9#. The line passing through this region indicates
the relationship between energy and angle for free p 1 -p
scattering.
Because of the lack of complete models describing the
pion absorption process ~especially for the heavier nuclei!, it
is difficult to fully correct the data in these figures for the
detector’s acceptance. Apart from any effects from the imR2591
© 1996 The American Physical Society
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D. ANDROIĆ et al.
53
FIG. 1. Scatter plots of laboratory kinetic energy vs angle from the pion beam axis for p p p
final states following the absorption of 239 MeV
p 1 on 3 He, 4 He, N, and Ar. The solid line shows
the kinematics for free p 1 -p scattering.
posed 30 MeV threshold, the largest missing correction is for
the geometrical acceptance of the detector. The losses are
thus small between 25° and 155°, but the acceptance falls to
zero smoothly over about 15° outside of this range. Structures visible in the figures at energies greater than 180 MeV
are largely artifacts of thresholds within the detector. For
further discussion of the acceptance and data extraction, see
@11#.
Figure 2 shows distributions as a function of energy for
the data appearing in Fig. 1 between 15° and 25°. For each
nucleus, the enhancement identified as ISI in Fig. 1 appears
in these spectra as a broad peak near the energy of the recoil
proton from free p 1 -p scattering, which is indicated by the
dotted line. The dashed lines on these plots are spectra of
protons resulting from simulations combining a uniform distribution over three-nucleon phase space with the detector’s
response, normalized to the data at the high-energy end of
the spectra. In these simulations, the residual nucleus momentum distribution was included by incorporating a weighting factor consistent with a spectator distribution. The phase
space spectra show that these peaks are not due to the detector acceptance.
In conclusion, evidence of the existence of ISI in pion
absorption has been shown, extending its direct observation
from 3 He to much heavier nuclei. In Ref. @9#, it was estimated that ISI directly accounts for about 30% of the yield
from 3 He when three protons share the energy of a 239 MeV
incident p 1 . Quantitative estimates for the heavier nuclei
require a more detailed analysis than presented here. However, the data shown in Fig. 2 indicate that the fraction of ISI
in the ppp final state does not greatly change between 3 He
and Ar.
FIG. 2. The laboratory kinetic energy of protons with polar
angles between 15° and 25° for ppp final states following the
absorption of 239 MeV p 1 on 3 He, 4 He, N, and Ar. The dashed
curves are the results of phase-space simulations, and the dotted
line shows the average proton energy for free p 1 -p scattering.
53
EVIDENCE OF INITIAL STATE INTERACTIONS IN . . .
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We thank the staff of the Paul Scherrer Institute for the
technical support provided to this experiment and P. Weber
for helpful discussions. This work was supported in part by
the German Bundesministerium für Forschung und Tech-
nologie, the German Internationales Büro der Kernforschungsanlage Jülich, the Swiss National Science Foundation,
the U.S. Department of Energy, and the U.S. National Science Foundation.
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